docs: cut CHANGELOG 0.4.0, reopen Unreleased

Everything under Unreleased goes out as 0.4.0: the mark now in use on the
README and in the panel (an addition, so a minor bump rather than a patch),
the signed-out pages' column width, the monitoring pages' width, and the
Deliveries subject/table-overflow fixes.

The heading is the version: there is no constant in the tree, only the VERSION
build arg that ldflags stamps into internal/buildinfo.Version, so the image
this deploys under is tagged from here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,8 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+## [0.4.0] - 2026-08-04
+
 ### Added
 
 - The project's mark is now in use rather than only on file. The README opens